In the dental industry, gels and pastes are utilized as vehicles for applying a variety of dentifrices, bleaching aids, remineralizing agents, and fluoride compounds to teeth. A gel is a colloid produced by combining a dispersed phase with a continuous phase (i.e. a dispersion medium or matrix) to produce a viscous, jelly-like, semisolid material. A “dental bleaching gel” is a gel that carries a bleaching agent that can be safely applied to teeth. Hydrogen peroxide and other peroxide producing sources have become the bleaching agents of choice for use in dental bleaching gels. Hydrogen peroxide is a powerful oxidizer, which serves to bleach the extrinsic and intrinsic chromogens in the teeth, thereby, producing a whiter appearance.
Viscosity is a very important parameter to control for effective dental bleaching gels, as it is a key determinant of peroxide release and in turn the whitening performance. Hydrogen peroxide is known to attack certain gelling agents and/or thickeners commonly used to make commercially available dental bleaching gels or pastes. For example, carboxypolymethylene thickeners conventionally used in whitening gels are susceptible to degradation by hydrogen peroxide under certain conditions. As a result of this attack, the gelling agents or thickeners break down over time; in some cases to such an extent that the gel's viscosity becomes too low to be suitable for use. Low viscosity gels flow uncontrollably from the dispensing tube, syringes etc. and become difficult to manipulate for their intended function. More importantly, if the viscosity is too low, the gel is more likely to flow away from the teeth, thus resulting in a reduced residence time and increased irritation due to undesired interaction of peroxide with soft tissues. Residence time is the time the dental bleaching gel actually is in contact with the tooth enamel.
Another problem associated with commercially available dental bleaching gels or pastes is that hydrogen peroxide tends to decompose at room temperature. The rate of this decomposition is dependent upon many factors. The presence of various metallic impurities, such as iron, manganese, copper and chromium, can catalyze the decomposition even when present in trace quantities. Furthermore, the stability of hydrogen peroxide decreases with increasing alkalinity and temperature, particularly in the presence of conventional thickeners such as carboxypolymethylene thickeners, in which case pH must be controlled by the use of pH buffers and the like. Because the whitening ability of a dental bleaching gel depends on the hydrogen peroxide concentration, premature decomposition diminishes the ability of the gel to whiten. Due to this instability, it has also been difficult to deliver other agents that can reduce sensitivity and increase remineralization.
One solution to these problems has been to refrigerate the dental bleaching gels or pastes until use. Refrigeration slows down the hydrogen peroxide attack on the gelling agent and also slows down hydrogen peroxide decomposition. However, refrigeration is both expensive and inconvenient. Various stabilizing agents have been investigated in an attempt to develop hydrogen peroxide containing dental bleaching gels and pastes that are stable at room temperature.
In view of the teachings of the prior art using conventional thickeners, there is a need for thermodynamically and chemically stable tooth whitening formulations that not only provide improved whitening but also provide a positive consumer experience.